Abstract
Abstract. Feldspar is an important constituent of airborne mineral dust. Some alkali feldspars exhibit particularly high ice nucleation (IN) activity. This has been related to structural similarities of the ice (101‾0) prism planes and the (100) planes of alkali feldspar. Here the effect of generating feldspar surfaces with close to (100) orientation by means of chemically induced fracturing on the IN activity of alkali feldspar was investigated experimentally. To this end, gem-quality K-rich alkali feldspar was shifted towards more Na-rich compositions by cation exchange with an NaCl–KCl salt melt at 850 ∘C. By this procedure, a system of parallel cracks with an orientation close to the (100) plane of the feldspar was induced. Droplet-freezing assay experiments performed on grain mounts of the cation-exchanged alkali feldspars revealed an increase in the overall density of ice-nucleating active site (INAS) density with respect to the untreated feldspar. In addition, annealing at 550 ∘C subsequent to primary cation exchange further enhanced the INAS density and led to IN activity at exceptionally high temperatures. Although very efficient in experiment, fracturing by cation exchange with an alkali halide salt is unlikely to be of relevance in the conditioning of alkali feldspars in nature. However, parting planes with similar orientation as the chemically induced cracks may be generated in lamellar microstructures resulting from the exsolution of initially homogeneous alkali feldspar, a widespread phenomenon in natural alkali feldspar known as perthite formation. Perthitic alkali feldspars indeed show the highest IN activity. We tentatively ascribe this phenomenon to the preferential exposure of feldspar crystal surfaces oriented sub-parallel to (100).
Highlights
Feldspar is the most abundant mineral in the Earth’s crust
The droplet-freezing temperature is a function of substrate activity, droplet footprint area, and cooling rate; the freezing behavior of different samples cannot be directly compared based on the freezing curves alone (Fig. 4a)
One of the possible explanations suggested for the repeatedly observed ice nucleation in the pores and cracks on the surface of alkali feldspars is the presence of small patches of crystal surface with (100) orientation that are exposed in the cracks due to natural fracturing or hydrothermal/deuteric alteration of alkali feldspar
Summary
Feldspar is the most abundant mineral in the Earth’s crust. It is a major constituent of magmatic, metamorphic, and sedimentary rocks (Smith and Brown, 1988), and due to its ubiquity on the Earth’s surface, feldspar is an abundant constituent of the solid aerosol particles. In light of the extraordinarily high IN activity of alkali feldspar and the preferential epitaxial nucleation of ice on feldspar (100) surfaces, it is important to understand the mechanisms by which such surfaces may form and to what extent the IN activity of these surfaces differs from the IN activity of the more commonly exposed growth facets and cleavage planes of alkali feldspar. The potential effects of different formation mechanisms and associated crystal surface morphologies on the efficacy of IN on alkali feldspar aerosol particles is of interest. In this communication we investigate the IN activities of alkali feldspars that were subject to different pretreatments designed to mimic natural processes leading to the exposure of crystal surfaces sub-parallel to (100). We relate the IN activity to the mode and the extent of the modification of the crystal surface by fracturing and discuss the potential role that (100) surfaces play for the IN activity of alkali feldspars
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